Re: [PATCH v4 02/14] memblock: add MEMBLOCK_RSRV_KERN flag

[Wei Yang]

On Wed, Feb 19, 2025 at 09:24:31AM +0200, Mike Rapoport wrote:
>Hi,
>
>On Tue, Feb 18, 2025 at 03:50:04PM +0000, Wei Yang wrote:
>> On Thu, Feb 06, 2025 at 03:27:42PM +0200, Mike Rapoport wrote:
>> >From: "Mike Rapoport (Microsoft)" <rppt@xxxxxxxxxx>
>> >
>> >to denote areas that were reserved for kernel use either directly with
>> >memblock_reserve_kern() or via memblock allocations.
>> >
>> >Signed-off-by: Mike Rapoport (Microsoft) <rppt@xxxxxxxxxx>
>> >---
>> > include/linux/memblock.h | 16 +++++++++++++++-
>> > mm/memblock.c            | 32 ++++++++++++++++++++++++--------
>> > 2 files changed, 39 insertions(+), 9 deletions(-)
>> >
>> >diff --git a/include/linux/memblock.h b/include/linux/memblock.h
>> >index e79eb6ac516f..65e274550f5d 100644
>> >--- a/include/linux/memblock.h
>> >+++ b/include/linux/memblock.h
>> >@@ -50,6 +50,7 @@ enum memblock_flags {
>> > 	MEMBLOCK_NOMAP		= 0x4,	/* don't add to kernel direct mapping */
>> > 	MEMBLOCK_DRIVER_MANAGED = 0x8,	/* always detected via a driver */
>> > 	MEMBLOCK_RSRV_NOINIT	= 0x10,	/* don't initialize struct pages */
>> >+	MEMBLOCK_RSRV_KERN	= 0x20,	/* memory reserved for kernel use */
>> 
>> Above memblock_flags, there are comments on explaining those flags.
>> 
>> Seems we miss it for MEMBLOCK_RSRV_KERN.
>
>Right, thanks!
> 
>> > 
>> > #ifdef CONFIG_HAVE_MEMBLOCK_PHYS_MAP
>> >@@ -1459,14 +1460,14 @@ phys_addr_t __init memblock_alloc_range_nid(phys_addr_t size,
>> > again:
>> > 	found = memblock_find_in_range_node(size, align, start, end, nid,
>> > 					    flags);
>> >-	if (found && !memblock_reserve(found, size))
>> >+	if (found && !__memblock_reserve(found, size, nid, MEMBLOCK_RSRV_KERN))
>> 
>> Maybe we could use memblock_reserve_kern() directly. If my understanding is
>> correct, the reserved region's nid is not used.
>
>We use nid of reserved regions in reserve_bootmem_region() (commit
>61167ad5fecd ("mm: pass nid to reserve_bootmem_region()")) but KHO needs to
>know the distribution of reserved memory among the nodes before
>memmap_init_reserved_pages().
> 
>> BTW, one question here. How we handle concurrent memblock allocation? If two
>> threads find the same available range and do the reservation, it seems to be a
>> problem to me. Or I missed something?
>
>memblock allocations end before smp_init(), there is no possible concurrency.
> 

Thanks, I still have one question here.

Below is a simplified call flow.

    mm_core_init()
        mem_init()
            memblock_free_all()
                free_low_memory_core_early()
                    memmap_init_reserved_pages()
                        memblock_set_node(..., memblock.reserved, )   --- (1)
                    __free_memory_core()
        kmem_cache_init()
            slab_state = UP;                                          --- (2)

And memblock_allloc_range_nid() is not supposed to be called after
slab_is_available(). Even someone do dose it, it will get memory from slab
instead of reserve region in memblock.

>From the above call flow and background, there are three cases when
memblock_alloc_range_nid() would be called:

  * If it is called before (1), memblock.reserved's nid would be adjusted correctly.
  * If it is called after (2), we don't touch memblock.reserved.
  * If it happens between (1) and (2), it looks would break the consistency of
    nid information in memblock.reserved. Because when we use
    memblock_reserve_kern(), NUMA_NO_NODE would be stored in region.

So my question is if the third case happens, would it introduce a bug? If it
won't happen, seems we don't need to specify the nid here?

-- 
Wei Yang
Help you, Help me